1. Field of the Invention
This invention relates to a confocal optical apparatus.
2. Description of the Related Art
Confocal optical apparatuses have image-forming characteristics superior to those of the conventional microscopes, and are employed in Scanning Laser Microscopes (SLM) or the like which have recently focused the spotlight of attention. An example of the confocal optical apparatuses is shown in FIG. 4. As is shown in the figure, a linearly polarized light beam such as laser beam emitted from a light source 12 is transformed into a parallel beam by means of a collimator lens 14, and then reflected by a polarization beam splitter 16. Subsequently, the reflected beam is converged onto a sample plane 22 by an objective lens 20. The light beam reflected from the plane 22 passes the objective lens 20, where it is again transformed into a parallel beam, and enters into the polarization beam splitter 16. Since the beam has passed a .lambda./4 plate 18 twice, the polarization of the light beam is rotated 90.degree., the beam passes through the splitter 16, and enters into an image-forming lens 24. The beam converged by the lens 24 is directed onto a shading plate 30 located at the focal point of the lens 24. The shading plate 30 has a pin hole 32. The beam having passed through the hole 32 enters into a photoelectric detector element 34. The beam from the plane 22 located at the focal point of the objective lens 20, travels along the optical path indicated by the solid line, and passes through a pin hole 32. On the other hand, a beam from the plane (indicated by e.g. a reference numeral 23) located out of the focal point of the objective lens 20, travels along the optical path indicated by the broken line, and almost all part of the beam is shaded by the shading plate 30 and hence does not enter into the photoelectric detector element 34. The element 34 provides an image-forming unit 36 with a signal indicative of the intensity of the received beam. A scanning unit 38 moves the sample, such that the beam spot is moved on the sample plane 22. The unit 38 supplies the image-forming unit 36 with a signal indicative of the position of the beam spot on the sample plane 22. The image-forming unit 36 forms an image of the sample plane 22 based on signals supplied from the photoelectric detector element 34 and scanning unit 38, and displays the image on a display unit 40. This image has a high contrast, i.e., a high resolution. Further, the optical apparatus has a resolving power along the optical path, and provides an image of a desirable plane of the sample, the operation is generally referred to as optical slicing.
In the above-described confocal optical apparatus, the image-forming lens 24 is generally constituted by a single convex lens. Referring now to FIG. 5, the diameter W2 of the airy disk is 50 .mu.m with the lens 24 having a focal length of 180 mm. The pin hole, which has the diameter in accordance with that of the airy disk, is used. As stated above, the diameter of the airy disk is small, the diameter of the pin hole to be used has to be small, so that positioning of the pin hole is difficult. Further, the confocal apparatus cannot be constructed such that an iris diaphragm is used instead of the pin hole, thereby continuously changing the diameter of the aperture of the diaphragm. In addition, where the lens 24 has a long focal length, the diameter W3 of the airy disk will be large, as is shown in FIG. 6. For example, where the focal length of the lens 24 is 5,000 mm, the diameter W3 will be approx. 1 mm. Accordingly, the pin hole having a large diameter may be used, so that optical adjustment can be performed easily. In this case, an iris diaphragm may be used as the pin hole. However, since the focal length of the lens 24 is long, the apparatus is inevitably large. In order to make the apparatus compact, if the optical path is bent by a plane mirror, the amount of light may be disadvantageously reduced due to a loss in reflection at the plane mirror.